Building a basement under propsed single storey extension

Planning a rear single storey extension and have had thought about creating a basement area under the proposed area I am extending out to.

What is the general thoughts on this? I guess the foundations will need to be dug further down and the ground area soil removed to the required depth. What type of structure will need to be built to comply with building regs?

What kind of cost saving would I expect to do this as part of the single storey extension - which will be beam and block floor.

My labour costs are more or less minimal as I have friends and family (in the trade) lending a helping hand.

300-330 solid walls, but will need designing and confirming suitable for the ground. Specialist waterproofing design will be necessary, not just any old architectural technician one-liner specification, and unless your free labour are experienced in waterproofing basements, then you may want to pay someone else.

Planning permission will be required.

I can confirm this is HARD WORK !! I've used a Newton 500 tanking system and it works. See what level your water table is at, as you might need a sump pump and drains to keep the area dry. Where you are connecting to your existing structure, I would not be surprised if you needed to dig under existing structure a little too. The Structural Engineer should explain how deep, and in what order any things need to be dug.

I've added some engineers drawings for where I underpinned an existing house...